It is known to use shock tube assemblies in order to simulate the static and dynamic pressure conditions resulting from large energy blasts. Radiant heaters such as oxygen-aluminum powder rockets are used in the shock tubes to simulate nuclear blast temperature conditions on the target. These large energy blasts may be the result of conventional explosive detonation or nuclear detonation. By simulating the conditions of such blasts without an actual full scale detonation, it is possible to evaluate the effects of such blasts on various types of equipment ranging from relatively small test articles such radios and the like, to relatively large test articles such as full size operational shelters, vehicles, tanks and aircraft. In effect, the shock tube assembly is a specialized short duration wind tunnel used for test and evaluation of various structures.
Typically, a shock tube assembly includes various sections, such as a driver section containing the pressurized gas which is ultimately used to create the shock wave, a diaphragm section to suddenly release the driver gas, an expansion nozzle section to port the driver gas into an expansion tube, along with associated gas processing and support equipment. The test article to be tested is placed in the test section of the expansion tube.
The driver is normally a hollow cylindrical pressure vessel with one end closed and sealed at the other end by the diaphragm section and capable of holding room temperature or elevated temperature gas at substantial pressure. The diaphragm section, associated with the driver, includes one or more diaphragms which are ruptured to release the gas in the driver, i.e., the shock tube diaphragm is mechanically, explosively or pressure ruptured to suddenly release the gas from the driver. In a dual diaphragm system, only one diaphragm is ruptured and the higher pressure differential imposed on the second diaphragm bursts it to release the gas. From the diaphragm section, the gas flows through the expander nozzle section, the discharge end of which is located within the expansion tube. The gas flowing through the nozzle section is supersonically expanded within the expansion chamber to create a shock wave which travels down the elongated expansion tube, compressing the air behind the travelling shock wave interface thereby providing both the static and dynamic pressure conditions and temperature conditions for testing and evaluating the test article located within the expansion tube and which is exposed to the static and dynamic pressure generated by the shockwave. Normally, one test article is tested in each firing and the test article usually is not larger than 10% of the cross-section of the expansion tube.
Shock tube assemblies may be of various sizes depending upon the blast conditions to be simulated and the test articles to be tested. For example, one such assembly for generating overpressures of about 200 psi, includes a driver section about 40 cm in length with an expansion tube having a length of about 100 cm and a diameter of about 7.5 cm. Such a system may use helium gas as the driver gas, the latter pressurized to about 1,345 psig at a temperature of 530 degrees R to generate a shock wave travelling at Mach number=3.5 with a 200 psi static overpressure in the expansion tube. Other shock tube systems may include one or more drivers of roughly 2 meters in diameter and having a length of from 43 to 93 meters. The expansion tube may be semi-circular having a diameter of 20 meters and a length of between 200 to over 300 meters. These larger systems are capable of generating the desired static overpressures of about 35 psi and shock waves which travel at supersonic speed in the expansion tube with driver pressures of 2,250 psig.
It is recognized that there is a well known relation between the driver pressure ratio (ratio of driver pressure to ambient pressure) required to produce a shock wave of a given shock pressure ratio (ratio of the pressure behind the shock wave and ambient pressure) for a given expansion ratio (ratio of nozzle area to expansion tube area). Thus, the pressure of the gas in the driver effectively controls the shock pressure ratio. A second factor is the temperature of the gas in the driver. Upon release and flow of the gas through the expander nozzle, a contact surface is formed between the generated and moving shock wave and the air in front of the shock wave. It is important that the gas static temperature on each side of the contact surface be the same, i.e., no contact surface temperature discontinuity. This simulates the real world in which an explosion induced shock wave races through the ambient temperature air to impinge on the target. Air on both sides of the shockwave is initially at the same temperature. Gas at rest has only one temperature which is called total temperature. The total temperature is a measurement of the energy in the gas at a given pressure. When some of the internal energy of the gas is used to accelerate the gas to a velocity, the total temperature of the gas remains constant and a stationary thermometer inserted into the flow would measure the initial total temperature. However, if a thermometer could be inserted into the gas stream so that it moved with the stream at the stream velocity, it would measure the static temperature which is lower than the total temperature. The relationship between total temperature and static temperature is defined by the following equation: EQU T.sub.o =T.sub.s * (1+(k-1) * M.sup.2)
where:
T.sub.o =Total temperature, degrees R PA1 T.sub.s =Static temperature, degrees R PA1 k=ratio of specific heats=C.sub.p /C.sub.v =1.4 for air PA1 M=Mach number
For M=2.66, T.sub.o /T.sub.s =2 and for the static temperature at both sides of the shock in the expansion tunnel to be equal, the driver gas must be heated to 1,040 degrees R if ambient temperature is 520 degrees R (60 degrees F.).
If the temperature of the gas on the expander side of the contact surface is higher than that on the other side, then the generated dynamic pressure will be lower than desired. If the temperature of the gas on the expander side of the contact surface is lower than that on the other side, then the dynamic pressure will be higher than desired. In either case, the test does not accurately simulate the blast conditions.
The temperature of the gas in the driver may be calculated such that on expansion, the temperature of the expanded gas is equal to that on the other side of the contact surface. Elimination of contact surface temperature discontinuity may be achieved by control of the temperature of the gas in the driver according the relationship in the example equation. Thus, it may be necessary, for example, to maintain the temperature of the gas in the driver as high as 700 degrees F.
The following table indicates some of these typical and representative relations calculated on the basis of a 600 kiloton nuclear detonation:
______________________________________ Shock Driver Driver Overpressure Overpressure Temperature psig psia Degrees R ______________________________________ 35 1727 1137 30 1507 1037 25 1249 947 20 1017 857 15 785 763 10 309 671 5 279 570 2 99 534 ______________________________________
It is therefore apparent that the design of the gas supply system is not separable from the design of the driver because of the dynamic coupling between the two when they are used together in the compression/heating cycle of shock tube operation.
Given the need to maintain the gas in the driver at an elevated temperature for proper dynamic pressure simulation, formidable practical, economical and structural problems are presented. For example, the use of external heater coils surrounding the driver unit is economically prohibitive because of the size of the driver unit and the power costs to heat such external units and because of the time required to change from one temperature to another for various test conditions. Even for smaller driver units, the power costs are impressively high relative to the physical size of the driver. The use of internal insulation to maintain the temperature of heated gas within an unheated driver is fraught with problems, not the least of which is the need for an insulating material which can be reliably fastened to the driver wall, which insulator is non-porous, and is capable of withstanding temperatures of the order of 700 degrees F. or more, as will be apparent from the following discussion. Such an insulation and attachment mechanism has not been found after lengthy investigation.
In shock tube assembly operation, the typical sequence is to initially charge the driver with a pressurized gas at the proper predetermined pressure and temperature level. Once fired, the internal pressure and temperature within the driver drops rapidly to atmospheric pressure and in some cases drops to a negative pressure. If an insulation is used which is porous, i.e., has pinholes, the initial pressurization causes the gas to travel through the pinhole to the interface between the driver internal wall and the insulation. Upon firing, the sudden drop in pressure causes the pressure at the interface to blow the insulation inwardly and generally results in total effective loss of the insulation. Thus, the driver must be re-insulated between each firing, an operation which is quite expensive. Tests have shown that if the insulation is porous as in conventional firebrick, the gas under pressure permeates the ceramic and causes it to literally explode when the driver pressure suddenly drops.
Even if the insulation is pinhole free and capable of withstanding relatively high temperatures, there is the problem of differential expansion between the metal wall of the driver and the insulation. Effectively what occurs is that at room temperature, the insulation may be bonded to the metal wall. The heavy driver wall expands due to the internal pressure at a greater rate than the insulation and bond, causing the insulation to part from the wall. Normally, the gas in the driver is rapidly discharged upon firing and the pressure within the driver may drop to minus 10 psig in one second. This sudden drop in pressure causes the unbonded insulation to part from the driver wall. Once the insulation has parted it is removed from the wall by pressure, gravity or aerodynamic forces and therefore is ineffective for later shots.
One possible alternative is to use a double lined driver in which the insulation is placed between the heavy outer metallic wall and a thin inner metallic wall. Typically such an insulation may be fire brick. Here, the problem is one of economics due to the relatively high cost of installing the fire brick, the cost of the inner wall and the additional cost of the larger, thicker outerwall. Another approach is to circulate the gas in the driver through an external heater assembly. Due to the high pressure, high density and high temperature of the gas within the driver, this alternative requires blowers which can produce mass flow rates necessary at the high temperatures and pressures. Such equipment is not commercially available and would be extremely costly to design and build.
Among the other problems is that of initially charging the driver with gas at the proper pressure and temperature. As a practical matter, the time needed to charge the driver cannot be too long. For a 20 meter diameter shock tube calculated charge times of 16 hours are not acceptable with uninsulated drivers and this is about the length of time it would take with large air compressors filling a building 150 feet square. Such a system suffers from the disadvantage of large pressure drops in the tube type external heater and an air compression system which would charge the driver in 15 minutes or less would be prohibitively costly. The 15 minute charging period had been calculated as the maximum charging time that could be used to fill an unheated driver with heated pressurized gas; however, recent calculations show that the charging time should probably be reduced to 5 minutes or less.
It is thus apparent that advantages exist in providing a driver capable of containing a gas at a relatively high pressure and temperature and which is capable of being charged very quickly with a heated and pressurized gas to be used in the shot.
It is also apparent, due to the relation between the temperature of the gas on one side of the contact surface and on the other side thereof, that a need exists for a gas charging system which accurately controls the temperature of the gas charging the driver while providing the proper pressure in a time sufficiently short to allow operation of the shock tube before heat transfer from the hot gas to the uninsulated driver wall cools the gas below the required temperature.
It is also desirable to provide a superheater and evaporator capable of rapidly heating a gas or liquified gas to an elevated temperature and in which the temperature of the output gas may be easily controlled, especially at high volumetric delivery rates.
It is also well known from simple thermodynamic relationships that it takes much more power to compress a weight of gas to an elevated pressure than it takes to pump a relatively incompressible liquid to that pressure and evaporate the liquid to gas at the elevated pressure. That is the way steam engines work and the boiler feed pump takes a relatively minuscule amount of power to supply pressurized water to a boiler which evaporates the water to steam which runs the steam engine that powers the pump and yields a net output. Similarly, it takes only a fraction of the power required by air compressors for piston type cryogenic pumps to raise the pressure of cryogenic liquid nitrogen to 2,250 psia and to pump it through a pebble-bed heater which evaporates it to produce hot gas at the required temperature.
It is also apparent that the provision of a gas charging system, in the form of an improved and effective pebble-bed heater, for a shock tube assembly which is relatively inexpensive and reliable offers unique advantages.